Electronic measuring devices such as a spectrum analyzer and an oscilloscope allow an electrical phenomenon which cannot be directly seen with human eyes to be seen as a waveform varying as a function of time. Therefore, they are widely used for observation of electrical phenomena. That is, these measuring devices can measure and evaluate the voltage values, current values, period, frequency characteristics, rising characteristics, and the like of input signals.
A description will be made especially on a case wherein a signal to be evaluated is a digital signal having multi-value voltage levels, e.g., a PCM (pulse-code modulation) signal.
As a technique of evaluating the characteristics or quality of a PCM signal, the degree of opening of an eye pattern is measured. More specifically, an eye pattern is obtained by synchronously sweeping a PCM signal using an oscilloscope and displaying signal waveforms to superpose them. An average amplitude level (a) and a minimum amplitude level (b) of the eye pattern are obtained by manual observation, and their ratio is calculated as a degree of opening.
In another technique, digital waveform analysis is applied to eye pattern measurement. More specifically, a plurality of waveform data are stored, and the levels of all the waveform are measured. Thereafter, the degree of opening of an eye pattern is automatically obtained by arithmetically processing the measurement results.
Since these conventional techniques involve manual observation, satisfactory reliability cannot be ensured in terms of measurement precision and resolution. In addition, in automatic measurement using the conventional digital waveform analysis technique, in order to ensure compatibility of measurement values obtained by automatic measurement with those obtained by manual observation, complicated processing is required. Hence, real-time measurement is difficult to perform, and high-speed measurement cannot be easily realized.
In order to eliminate the above-described drawbacks, a demand has arisen for an accurate, simple measurement technique. A great deal of attention has been paid to a technique for evaluating the characteristics of an input signal by detecting the leading and trailing edges of the input signal, i.e., a waveform in a transition state. That is, the inclinations of an input signal, which rises and falls, are read as a waveform, and data processing is performed later. In this technique, however, the characteristics of an input signal cannot be quantitatively evaluated in a real-time manner.